1. Field of the Invention
The present invention relates to an alignment apparatus (positioning apparatus) arranged in a projection exposure apparatus which projects exposure light passing through a medium with projection information, such as a photomask or a reticle, onto a semiconductor wafer or a print-circuit board through a projection optical system to transfer the information thereon, for relative positioning of the photomask or the reticle at a transfer position with high precision with respect to the semiconductor wafer or print-circuit board.
2. Related Background Art
The projection exposure apparatus is used for fabrication of semiconductor elements or liquid crystal display devices, applying the photolithography technology. This projection apparatus supplies exposure light, which passes through a photomask or reticle and then irradiates a substrate (semiconductor wafer, glass plate, etc.) coated with a photosensitive material through a projection optical system to transfer projection information thereonto. For example in case of fabrication of semiconductor elements (IC, LSI, etc.), a plurality of exposure processes are conducted to form a plurality of layered or overlaid circuit patterns on a semiconductor wafer. Further, to produce semiconductor elements with high yield, it is necessary to control the overlay accuracy (positioning accuracy) within a certain permissible range between a circuit pattern in a previous layer formed on the semiconductor wafer and a next circuit pattern successively transferred. In order to improve the positioning accuracy, the projection exposure apparatus is provided with an alignment apparatus for relative alignment between the photomask or reticle and the semiconductor wafer.
Among conventional alignment apparatus, there is an apparatus expected as one capable of theoretically achieving the highest accuracy, which is of a TTR (through the reticle) type in which a relative position is directly adjusted between the reticle and the wafer through the projection optical system. The alignment apparatus must use alignment light with a wavelength different from that of the exposure light to avoid exposure of the coating photoresist on the wafer by the alignment light. However, in case the wavelength of alignment light is different from that of exposure light, the projection optical system causes chromatic aberration for the alignment light, which was a problem. Japanese Patent Publication No. 1-40490 and U.S. Pat. No. 5,100,237, U.S. Pat. No. 5,160,849, U.S. Pat. No. 5,204,535 disclose alignment apparatus arranged to correct the chromatic aberration.
First, the alignment apparatus as disclosed in U.S. Pat. No. 5,100,237 has a correction lens located at the center of optic axis and at the entrance pupil of projection optical system to correct the chromatic aberration between the wavelength of exposure light and the wavelength of alignment light. In such arrangement, alignment is carried out while detecting.+-.first-order diffracted light from an alignment mark (wafer mark) on the wafer.
The alignment apparatus as disclosed in Japanese Patent Publication No. 1-40490 has a correction optical system located outside or inside an exposure light path between the reticle and the projection optical system to correct the chromatic aberration of alignment light as caused in passing through the projection optical system. The apparatus detects a wafer mark and an image of reticle mark (alignment mark on reticle) formed on the wafer through the projection optical system, whereby alignment is effected between the reticle and the wafer.
The above-described alignment apparatus as disclosed in U.S. Pat. No. 5,100,237 employs the correction lens for correction of chromatic aberration located at the center of entrance pupil of projection optical system and sized small enough to avoid an adverse effect on the exposure light. Meanwhile, there is a need to make finer the pitch of diffraction grating as the wafer mark to further improve the alignment accuracy. Then, decreasing the pitch of diffraction grating as the wafer mark results in increasing a separation between the.+-.first-order diffracted light as detection light at the entrance pupil of projection optical system, which requires a larger correction lens. As a result, the influence of correction lens on the exposure light cannot be negligible and the apparatus is not then suitable for applications requiring higher accuracy of alignment.
Furthermore, depending upon steps in alignment mark or the conditions of duty ratio, the intensity of.+-.first-order diffracted light becomes very low. In that case, use of higher-order diffracted light is effective. However, the size of correction lens must be increased to use the higher-order diffracted light and therefore the influence of the correction lens on the exposure light cannot be negligible. Therefore, the alignment apparatus as disclosed in U.S. Pat. No. 5,100,237 was unable to utilize the higher-order diffracted light.
The another alignment apparatus as disclosed in Japanese Patent Publication No. 1-40490 is theoretically capable of correcting the axial chromatic aberration of projection optical system by setting the correction optical system outside or inside the exposure light path between the reticle and the projection lens. The apparatus, however, had such a problem that an image of wafer mark dropped in the exposure area on reticle because of the lateral chromatic aberration of projection optical system when the image of wafer mark was seen through the projection optical system, because the wavelength of alignment light was longer than that of exposure light. It is conceivable in order to avoid this problem that a plane-parallel plate is arranged with an adjustable angle of inclination outside the exposure light path between the reticle and the projection optical system to correct the lateral chromatic aberration of projection optical system and to shift the image of wafer mark out of the exposure area. This method is not satisfactory in that the plane-parallel plate shields a part of exposure light.
Moreover, it is very difficult in respect of designing and production to achieve a projection optical system having no chromatic aberration (axial chromatic aberration and lateral chromatic aberration) for the both light beams (exposure light and alignment light) of different wavelengths in order to solve these problems. Especially, an exposure apparatus using an excimer laser source as the light source of exposure light has chromatic aberration several ten times greater than that in an exposure apparatus with exposure light which is a bright line (g-line, i-line, etc.) from a mercury lamp, because materials which can be used for the projection optical system are limited to quartz, fluorite or the like.